1. Field of the Invention
The present invention relates to solid fuel propellant motors and in particular to means for reducing the hazard of unwanted ignition of such motors due to slow extraneous heating.
We refer to our copending patent application number PCT/GB96/00325 the content of which is included herein by reference. In particular, this reference describes a device to mitigate the effects of unwanted extraneous heating under so-called "slow cook-off" conditions.
2. Discussion of Prior Art
Explosive materials can be broadly divided into those which are intended to detonate and those which are intended to ignite or burn, albeit very rapidly. The present invention is mainly concerned with the latter type of material, an example of which is propellant material used for applications such as rocket motors.
Propellant materials, based for example on nitrocellulose (NC) or mixtures of nitrocellulose and nitroglycerine (NG), when subjected to extraneous heating at a relatively rapid rate will generally react by producing an ignition event where the material burns rather than detonates. The temperature at which this ignition event occurs is dependent on the heating rate of the material. A typical ignition temperature range for propellants based on cast NC/NG matrices is about 160 to 180.degree. C. at, for example, a heating rate of 5.degree. C. per minute. However, although undesired ignition of a propellant motor is a serious and dangerous event, the potentially more dangerous event of explosive detonation may occur when the material is subjected to a very low heating rate. A heating rate of about 0.05.degree. C. per minute, for example, from ambient temperature may cause detonation at a temperature of about 120.degree. C.
During a detonation event, substantially all molecules of the material release their chemical energy simultaneously. A slow heating rate serves to bring the majority or all of the molecules in the motor matrix to an energy level where the next increment of heat input takes the material above an activation energy "barrier" to promote a simultaneous reaction causing a detonation event.
Under conditions of low heating rate it is desirable to be able to stimulate an ignition at a lower energy level and cause the material to burn prior to detonation taking place. However, even the burning of a solid fuel propellant motor in a restricted environment is an extremely hazardous event. The rate of burning of a solid fuel propellant is very high and is dependent, inter alia, on the gas pressure and temperature conditions within the motor easing. Normally, a solid fuel motor may be fully consumed within a few seconds. However, if the same fuel is ignited under normal atmospheric pressure without the pressure being allowed to increase substantially, the rate of burning is very much slower, and consequently, much less hazardous.
EP-A-0334731 describes a device using a low melting point alloy effectively as an adhesive between two coaxial surfaces, one surface on a motor body and the other surface on a closure. On heating the low melting point alloy melts and allows ejection of the closure by springs acting in the axial direction. However, the low melting point alloy does not provide sufficient strength to retain the closure during normal firing operation of the motor. Consequently, an additional locking ring is needed that must be rotated into position prior to firing and thus adding unnecessary expense, complexity and potential unreliability to the device. Furthermore, the low melting point alloy must be cast in-situ thus further increasing cost and complexity of manufacture.
U.S. Pat. No. 5,311,820 describes a device whereby a motor closure may be retained in a motor body. This device employs two sections of overlapping tube having aligned corresponding grooves on the outside of the inner tube and on the inside of the outer tube, there being an internal spring retainer provided to lock the two tube components together. The groove in the outer tube is sufficiently deep so as to completely accommodate the spring retainer and so not engage the groove in the inner tube. Grub screws are used to force the spring retainer into the inner groove, and so cause the two tubes to be locked together. One by one the grub screws are removed and molten low melting point alloy cast in the vacant holes and allowed to solidify. When all the grub screws have been replaced, the key is maintained in the engaged position only by the low melting point alloy. On extraneous heating the low melting point alloy melts and the spring retainer springs out completely into the groove in the outer tube thus releasing the closure. This device has three major engineering disadvantages. The first is that the spring retainer ring is unlikely to move outwardly and submerge into the outer groove in a consistent and reliable manner such that a clean separation of the closure member is unlikely to occur. Secondly, the device increases the necessary outer diameter of the motor which is undesirable. Thirdly, the radial ejection of the low melting point alloy on melting means that the motor cannot be contained within a continuous outer skin or air frame. Fourthly, the necessity of sequentially removing grub screws and casting low melting point alloy into the holes is time consuming and adds to the cost of the device.
U.S. pat. No. 5,035,181 is specifically for use for fuses on bombs and shells. The device described can only be used where the shock wave propagates faster than the speed of sound, and therefore, has no relevance to pressure vessels such as rocket motor bodies.